Polyester-polyurethane hybrid resins are well-known in the art of thermoset molding compositions. These resins are normally tougher than polyesters and stronger, stiffer and less expensive than polyurethanes. Such hybrid resins usually comprise a hydroxy-terminated unsaturated polyester polyol, an ethylenically unsaturated monomer such as styrene, a polyisocyanate, a free radical polymerization catalyst and often other additives. They can be easily adapted to many common thermoset molding techniques employed in polyurethane and unsaturated polyester industries. Such hybrid resins are commercially available from Amoco Chemical Company under the trademark Xycon hybrid resins and are supplied as two component systems having an A and a B side. The A side contains the polyisocyanate and the free radical catalyst, while the B side contains the hydroxy-terminated unsaturated polyester polyol/styrene solution and optionally a polyurethane catalyst and/or filler.
It is known that sheet molding compounds (SMC) can be made from polyester or polyester-polyurethane hybrid resins. For example, a polyester or polyester-polyurethane resin paste which may contain a filler material such as calcium carbonate is first prepared and pumped into a sheet molding compound machine where it is applied to one or both of two carrier film sheets by metering blades and moved in two different directions. Optionally, the resin-coated sheets are moved past a station where metered amounts of reinforcing material such as chopped glass fibers are dropped onto the resin paste. Next, the sheets are typically joined together in a "sandwich" with the paste and glass-coated side of one or both of the two sheets as the center of the sandwich. The SMC is then moved through compression rollers to further impregnate the glass into the resin. This is called "wetting" of the glass fibers. The SMC is then removed from the SMC machine in rolls and transferred to a maturation room where the viscosity of the SMC is monitored. The initial viscosity of the SMC is low, i.e., about 15,000 to about 100,000 cps. As the SMC is matured or allowed to chemically react, the viscosity increases to much higher levels, i.e., about 30,000,000 to about 100,000,000 cps. The SMC is then taken to a molding station where it is unrolled, cut and separated from the carrier film. The SMC is placed in a mold, compressed under high pressure and cured.
One use of sheet molding compounds is to manufacture parts for automotive applications. However, in order to achieve the required strength for automotive parts, a reinforcing material such as chopped glass must be incorporated in the SMC as described above.
One problem associated with the manufacture of SMC with polyester-polyurethane hybrid resins is controlling the viscosity of the SMC paste at the initial stage so that the paste can be pumped into the SMC machine and a good wetting of the reinforcing fibers can be attained. Another problem is controlling viscosity at the maturation stage so that the viscosity of the SMC will increase but not to a level where it cannot be further processed into a molded part. Yet another problem is controlling the final properties of the molded part so that it will not be too brittle, will have a good surface appearance and will perform its intended function.
The general object of the present invention is to provide sheet molding compounds capable of improved viscosity control. It is another object to provide a method for making such sheet molding compounds. Other objects appear hereinafter.
These and other objects are achieved by improved sheet molding compounds which comprise polyester-polyurethane hybrid resins formed by the reaction of an A side composition and a B side composition. The A side composition comprises a polyfunctional isocyanate compound and a free radical polymerization catalyst. The B side composition comprises a mixture of (i) an ethylenically unsaturated monomer solution having no active hydrogen groups which has dissolved therein about 40-90 weight percent of a substantially water-free, unsaturated polyester polyol having at least one dicarboxylic alkene moiety and having an acid number less than five; and (ii) a methacrylate ester of a C.sub.2 -C.sub.10 alkylene diol.
For the purpose of this invention, the term "cure" or "curing" means the transformation of the sheet molding compound from a liquid or flowable paste to a solid cross-linked material at the time of molding. This curing occurs by cross-linking of the reactive sites in the sheet molding compound hybrid resin system, including the reaction of the isocyanate with active hydrogen-containing compounds. Further, the curing of the sheet molding compound occurs via the vinyl addition reaction between the ethylenically unsaturated monomer containing no active hydrogen groups and the unsaturated polyester polyol. Depending on the catalyst employed, curing can optimally occur at temperatures of about 135.degree. C. to about 160.degree. C. for a time of between about 30 sec. and about 5 min. The term "polyfunctional" and the prefix "poly-", as used herein, are intended to include functionalities of two or greater.